The present invention relates to image forming apparatuses and methods for performing image forming using an electrophotographic process.
Generally, in image forming apparatuses and methods for performing image forming using an electrophotographic process, toner is transferred on a toner carrier to form a patch image (reference image) and the amount or position of the toner on the patch image is detected to perform density or color misregistration correction. For example, in a mode (calibration mode) for appropriately setting the density of an image of each color in full-color image forming apparatuses, image forming units for cyan, magenta, yellow, and black are used to form a correction patch image of each color on transfer belts. The formed correction patch image is detected to perform density or color misregistration correction.
The density of black toner mainly relates to saturation and brightness, in the case of monochrome printing using only the black image forming unit, and the density of color toner mainly relates to hue in the case of color printing using cyan, magenta, and yellow image forming units. In addition, the saturation and brightness that are varied do not lower the impression of the image, compared with the situation where the hue is varied. Accordingly, it is not necessary to frequently correct the monochrome (black) toner density, compared with correction of the color (cyan, magenta, and yellow) toner density.
Since calibration of the respective colors is generally simultaneously performed based on a single correction condition relating to, for example, the number of prints or the printing time in such density correction in the color image forming apparatuses, correction of the monochrome toner density is performed too frequently.
In order to resolve the above problems, for example, in an image forming apparatus, the correction condition of the monochrome toner density is separately set from that of the color toner density based on the number of prints. And the interval of the number of prints corresponding to the time when the monochrome toner density is corrected is set to a value that is more than the interval of the number of prints corresponding to the time when the color toner density is corrected, to reduce the number of times when the monochrome toner density is corrected.
However, for example, if a large volume of monochrome printing is continuously performed even if the monochrome correction condition is set so that it is different from the color correction condition, the color toner density is simultaneously corrected with the correction of the monochrome toner density even without the color printing. If such a situation is repeated, the color toner in the developer is exposed to excessive mechanical stress accelerating deterioration of the toner. In addition, the amount of electrostatic charge of the toner may increase causing a condition wherein optimal toner density is not achieved in the color printing. Furthermore, the density correction is generally set so as to be forcedly performed by interrupting the printing process during continuous printing. Accordingly, if the correction condition is met during continuous printing, printing will be interrupted making the user wait for completion of the density correction, thus decreasing printing speed.
In order to resolve the above problems, in another image forming apparatus, the time when the monochrome toner density is corrected and the time when the color toner density is corrected are appropriately set. Control means for separately performing correction of the monochrome toner density and of the color toner density is provided to reduce the number of times when unnecessary correction of the color toner density is performed, thereby reducing the deterioration of the color toner and maintaining stable image density.
However, since correction of the monochrome toner density and of the color toner density are performed based on the number of prints or the printing time in principle, density correction may be performed even if there is no need to perform the density correction in the image forming units. Particularly, density correction that occurs during monochrome printing interrupts printing by the user although it is often not necessary to perform the density correction, that is, only a small amount of correction is often achieved by density correction in monochrome (black) printing.